3 Answers
3

SLR - Single Lens Reflex, meaning that there is only one lens through which the viewfinder and the film are exposed to the image. This is in contrast to other types of cameras, like rangefinders, where the viewfinder was a separated optical path than the film, usually in the form of a small lens on the top-side of the body. The Reflex part comes from the rotating mirror which is used to direct the image from the lens to the viewfinder. When the shutter is released, first the mirror goes up and clears the optical path to the film.

Another characteristic of this class of cameras is interchangeable lenses. I am not sure, though, if historically all SLR's had this option, and probably some of our experienced forum members can comment on that.

Nowadays, with the DSLR technology, sensors replaced the film but otherwise the basic structure and principles remained the same.

The Fuji camera in your link is not considered an SLR for the lack of the mirror, pentaprism and optical viewfinder. There, it is an electronic viewfinder, meaning that what you see is an image generated on a tiny LCD inside the viewfinder assembly.

Yes. Not all SLRs had interchangeable lenses. There was also some models using a beam-splitter rather than a mirror, similar to Sony SLT models, but with an OVF.
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ItaiFeb 3 '11 at 21:19

So, if I understand correctly, the Fuji HS 10 does let you see (even if it's not using the mirror system) the exact framing of the picture, right? What's the downside of this compared to SLR?
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cambracaFeb 3 '11 at 21:22

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@cambraca - Yes, basically you see what the sensor sees. Problem is that the VF LCD is not top-quality and the image is prone to all digital processing artifacts. The resolution is relatively low and the image processing is limited so the image looks evidently "digital" (speaking from my experience with the Canon S1IS and S5IS, can't comment on the Fuji directly). The SLR lets you see the "real thing".
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ysapFeb 3 '11 at 21:30

@cambraca - as you notice, many compact cameras today gave away the viewfinder in favor of the back LCD. The principle is similar, but the VF lets you compose in bright sunlight.
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ysapFeb 3 '11 at 21:32

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+1 - Well answered. It's important to note that the 'R' in SLR is for the reflex mirror assembly, so by definition, a camera without the reflex finder cannot be an SLR even if everything else is pretty much the same.
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John Cavan♦Feb 4 '11 at 0:37

Single lens. That is, it does not have a separate lens for a viewfinder. If it has a viewfinder, the viewfinder looks through the main lens.

R

Reflex. This means it has a mirror (called a "reflex mirror") that allows light from the main lens to be bounced upwards onto a ground glass screen. Modern SLRs usually then use a pentaprism or pentamirror above this ground glass screen to allow viewing the image up the right way and from behind.

SLR is a term that's pretty old now, and at the time distinguished itself from other camera designs which used a separate lens assembly for an optical viewfinder, such as rangefinder cameras or twin lens reflex cameras.

Benefits and drawbacks

The benefit of having the optical viewfinder look through the primary lens is that you can change that lens and still see what the camera would see, without having to also change or somehow adapt the viewfinder lens.

The drawback is that the drop-down mirror assembly means there has to be a fairly long distance between the lens flange and the focal plane (film or sensor), which restricts lens designs, particularly making wide-angle lenses bulkier and more complicated.

The Fujifilm Finepix HS10

This passes on the SL part, but fails on the R part. It has no mirror bouncing the image up to a ground-glass screen. This is because it has no optical viewfinder at all, and even if it did it wouldn't have to be a through-the-lens one since the lens is not interchangeable.

The non-interchangeable lens and small sensor designate this as a compact camera; though from looking at it clearly it is not very "compact" in the literal sense. These types of compacts are often called "superzooms" because of their large zoom range, and to distract you from the fact that for a compact camera they're not very compact. You may also get away with calling them bridge cameras - though this is a misleading marketing term designed to give the impression it is somehow better quality than a normal compact camera. The only way in which these resemble an SLR is in size and weight - the sensor and optics are still those of a compact.

Nowadays, there is increasingly less need for an optical viewfinder even on interchangeable lens cameras, since electronic viewfinders offer some unique benefits such as zoomed image previews, histograms etc, and their image quality is improving lessening the gap with a good ground glass screen (not that the Finepix HS10's electronic viewfinder is any good). That's why you are starting to see "mirrorless", yet through-the-lens, systems such as Micro Four Thirds and Sony Nex entering the market. They can be made smaller and with a smaller lens flange to sensor distance, the lenses (particularly wide angle lenses) can be made smaller and lighter too.

So far, all the mirrorless systems also use a smaller sensor size than full frame making DSLR (or rangefinders) the only options for those needing the characteristics of full-frame sized sensors. Also, mirrorless systems typically use contrast detect auto focus (CDAF) instead of phase detect auto focus (PDAF). Historically CDAF tends to be slower and PDAF tends to be less accurate but this is only a generalisation; CDAF is improving in the speed department and an expensive PDAF system will be accurate anyway.

The benefit of SLR design is to get rid of parallax error of TLR (Twin Lens Reflex) and rangefinder cameras. That SLR design also makes it easier and cheaper to be able to have interchangeable lenses is a side-effect.
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Esa PaulastoAug 12 '13 at 6:31

One person's side effect is another person's main goal. I would think that for the most part, having interchangeable lenses is not a nice-to-have bonus but a central feature of an SLR, while removing parallax error is the side bonus. But others may indeed see it differently.
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thomasrutterSep 26 '13 at 3:23

To-date, the moving mirror & viewfinder mechanism that @yasp described is sort of the defining characteristic of an SLR. I think we're going to see this "clean" definition continue to erode, though, with new cameras on the horizon.

Other characteristics generally attributed to the DSLR format:

Larger sensor size than compacts or bridge cameras

Interchangeable lenses

Optical viewfinders

There are already Electronic Viewfinder Interchangeable Lens (EVIL) cameras such as Micro 4/3 cameras, that deliver many of the benefits of DSLR's without meeting this specific definition of DSLR, and more are on the horizon. The new Sony A55 and A33, for instance, use a translucent mirror that doesn't move, and Nikon is rumored to be introducing a "pro" mirrorless camera soon, too.

As more of these "not quite DSLR" cameras enter the market, I wouldn't be surprised to see the traditional definition of DSLR become less important, if not less clear.

A pellicle (translucent) mirror camera is still a "proper" SLR. The Sony models aren't the first -- the Canon EOS RT (for "real time") was a 35mm film SLR with a pellicle mirror that could do 30 fps at the X-sync shutter speed or faster and had a single-shot shutter lag that was shorter than the then-current Leica rangefinder.
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user2719Feb 4 '11 at 3:29

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"I wouldn't be surprised to see the traditional definition of DSLR become less important": Incidentally, in Finnish, we do not usually use the term SLR, but we use the term "system camera", which can be applied to both DSLRs and EVILs.
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Jukka SuomelaFeb 4 '11 at 11:22

@Stan - thanks for the clarification. It'll be interesting to see how Nikon approaches this if their rumored mirrorless camera comes to pass.
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D. LambertFeb 4 '11 at 13:33

@Jukka - that makes sense. I guess my tangential answer here was meant to suggest that although there's a correct technical answer to this question, it's also helpful for people to understand the benefits that we've come to expect from a DSLR so that they can see which of those benefits are (or aren't) available in other systems.
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D. LambertFeb 4 '11 at 13:36